ON THE TBANSMISSrON OF POWER BY COMPRESSED AIR. 453 



indicated horse-power per revolution per minute. The mechanical effi- 

 ciency was sensibly the same at all speeds, viz. 84-5 per cent., as given in 

 the Table. There was no method available for ascertaining to what extent 

 the real quantity of air delivered corresponded to the nominal volume 

 swept through by the compressor pistons. The indicator diagrams (Pig. 2) 

 show no signs of leakage past the valve, but there are no doubt various 

 possible leakages which would not show on the diagrams. In the absence 

 of any direct means of determination, however, I have assumed that the 

 compressor cylinders delivered their full volume, which corresponds to 348 

 cubic feet' of air per indicated horse-power per hour. This air has a 

 weight of about 25 lbs. It may be pointed out that the water injected 

 practically fills up the clearance space at the end of each stroke. 



I have already pointed out that, at whatever temperature the air is 

 delivered, it must fall to about its original temperature in the long length 



SCALE BO LBS HER INCH 



of mains before it reaches the motors. It is therefore a simple matter to 

 find the maximum amount of work which can be done by the air delivered 

 per indicated horse-power, for it simply amounts to the P V of the air at 

 six atmospheres absolute and at 70°. Fahr. plus the work it can do in 

 expanding adiabatically to a pressure of one atmosphere, and minus the 

 work necessary to expel it from the cylinder at that pressure and at the 

 corresponding temperature.^ In the present case I find that this work is 

 equivalent to 0-52 indicated horse-power for an hour, so that the eflBciency 



' Here and elsewhere, unless specially mentioned, volumes are supposed to be at 

 atmospheric pressure and at 70° Fahr., the actual admission temperature during my 

 experiments. 



" In symbols, if suffixes 1 and 2 be used for the initial and final conditions of the 

 air, if pressures be measured in pounds per square foot and volumes in cubic feet, 

 the maximum work possible, without addition of heat, is— 



(P, V, - P,V,) -f-^ = 3-45(P,V, - P,V,). 



